KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 449 



B. We may then see whether this testimony can be checked or controlled by any 

 more direct evidence, drawn from other sources ? 



A. Testimony of the Colour-Blind. 



It is a matter of notoriety that the intercourse of the colour-blind with the world in 

 general leads them to speak of yellow and blue as the two colours about which they have 

 least doubt ; and when any intelligent person of this class is made aware of the general 

 nature of his defect, he holds a strong opinion that these are the two colours he sees. 

 This was the case with myself, as I stated in my paper of 1859 ; adding that 

 " the pigments ultramarine and chrome yellow, or the parts of the solar spectrum near 

 the lines D and F, excite the colour-sensations I am capable of, most fully and com- 

 pletely." 



But this idea was called in question on theoretical grounds. Professor Clerk 

 Maxwell had interested himself to publish and explain what is now called the Young- 

 Helmholtz theory of colour-vision, which assumed that there were three elementary 

 colour-sensations, usually taken as red, green, and blue or violet, by the excitement and 

 mixture of which all normal colours might be produced. It was natural to apply this to 

 colour-blindness by assuming that dichromic vision was caused simply by the absence of 

 one of the three sensations, the two others remaining intact. Professor Maxwell was good 

 enough to try many experiments on my vision, which he regarded as confirming these 

 theroretical views ; and, on the 24th March 1871, he delivered, at the Royal Institution, 

 a lecture on Colour- Vision, in which he made the following remarks : — 



The defect consists in the absence of one of the three primary sensations of colour. Colour- 

 blind vision depends on the variable intensities of two sensations instead of three. The best descrip- 

 tion of colour-blind vision is that given by Mr Pole in his account of his own case in the Phil. Trans., 

 1859. 



In all cases which have been examined with sufficient care, the absent sensation appears to 



resemble that which we call red We have great reason, therefore, to conclude that the 



colour-sensations which Mr Pole sees are what we call green and blue. 



This is the result of my calculation, but Mr Pole agrees with every other colour-blind person 

 whom I know in denying that green is one of his sensations. The colour-blind are always making 

 mistakes about green things, and confounding them with red. The colours they have no doubt about 

 are certainly blue and yellow. 



Professor Maxwell, however, went on to explain what he meant. He did not intend 

 to deny that, from a practical point of view, our sensation of the warm colour was the 

 same as was given us by the normal yellow, — for he had said expressly (Paper of 1855, 

 p. 287), as the result of his mathematical investigation, " Hence all colours appear to 

 the colour-blind as if composed of blue and yellow." He believed that, according to his 

 theory, our warm hue was due to the green primary sensation, — but he admitted that, by 



VOL. XXXVII. PART II. (NO. 22) 3 Z 



